1. Field of the Invention
This invention relates to heat radiators (or heatsinks) that remove heat generated by electronic devices, which include semiconductor elements mounted on substrates (or boards) or installed in casings, by heat conduction or radiation. In addition, this invention also relates to methods of manufacturing the heat radiators for dissipating heat from the electronic devices to the outside. Further, this invention relates to semiconductor laser modules in which the heat radiators are attached to bottom walls of packages for providing semiconductor laser elements.
2. Description of the Related Art
Advances in recent technologies bring high power outputs and high integration of electronic devices such as semiconductor elements, which may correspond to thermoelectric elements, integrated circuits (ICs), large-scale integrated circuits (LSI), very-large-scale-integrated circuits (VLSI), and diodes, for example. There is also provided the tendency to rapidly increase amounts of heat generated by electronic devices. For this reason, semiconductor devices such as hybrid ICs made by high integration of circuit elements employ heat radiation systems (or heatsink systems) for efficiently dissipating heat of semiconductor elements to the outside. In practice, heat radiator boards (or heatsink boards) made of copper (Cu) or other metal materials having high melting points are integrally joined to ceramic circuit boards. In general, large differences in coefficients of thermal expansion exist between the heat radiator boards made of copper or other metal materials having high melting points and the semiconductor elements or between the heat radiator boards and the circuit boards. Due to thermal shocks that are repeatedly effected on the semiconductor devices because of the large differences of thermal expansion coefficients, thermal stresses are increased at joint interfaces between the heat radiator boards and semiconductor elements (or circuit boards), so that the heat radiating boards are easily separated therefrom.
Because of the aforementioned difficulty, it is necessary to develop heat radiator boards whose thermal expansion coefficients approximate those of the semiconductor elements or circuit boards for practical use. That is, the heat radiator boards are made from sintered bodies composed of tungsten (W) and other metal materials having high melting points. However, the heat radiator boards made of only the tungsten or other metal materials having high melting points may be insufficient in thermal conductivity. Hence, manufacturers have developed new heat radiator boards made of impregnated sintered alloys that are made by infiltrating (or impregnating) high thermal conductive materials such as copper (Cu) into vacancies (or cavities) in the sintered bodies made of only the tungsten or other metal materials having high melting points.
The aforementioned heat radiator boards made of impregnated sintered alloys are generally manufactured by the following steps.    (i) Powders of high melting point materials such as tungsten (W) are subjected to preliminary blending together with organic binders, so that a material mixture is produced and is subjected to metal mold pressing to form a thin board.    (ii) The formed thin board is subjected to degreasing and sintering to produce a porous sintered body including vacancies, into which high thermal conductive materials such as copper (Cu) are infiltrated (or impregnated).    (iii) Thereafter, the surface of the impregnated sintered body is subjected to surface processing using the milling machine (fraise) or lapping machine. Thus, it is possible to manufacture the heat radiator board.
In order to improve a thermal conductivity of the heat radiator board formed by the impregnated sintered body, it is necessary to increase the content of copper (Cu) which is superior in thermal conductivity. If such copper content is increased very much, the overall thermal expansion coefficient of the heat radiator board is increased. Under the influence of thermal shocks repeatedly caused, thermal stresses should be increased at joint interfaces between the heat radiator boards and the boards for mounting electronic devices or at joint interfaces between the heat radiator boards and the casings for installing electronic devices. This causes the heat radiator boards to easily peel off (or be removed) from the boards or casings.
In the heat radiator board made by the impregnated sintered body, copper materials are infiltrated (or impregnated) into vacancies that are formed inside of the sintered body. Hence, the thermal expansion occurs in such a manner that the thermal conduction progresses along prescribed portions at which the copper materials are impregnated. This indicates that the thermal conduction is made in random directions. Suppose that the heat radiation (or heat dissipation) occurs in random directions from the heat radiator board which is attached to the casing for installing electronic devices. In this case, it becomes difficult to promptly dissipate heat, generated by the electronic devices, to the outside of the casing. That is, there is a problem that the heat radiation efficiency (or heat dissipation efficiency) is deteriorated.
In addition, the heat radiation board made by the impregnated sintered body requires infiltration (or impregnation) of copper materials into vacancies of the sintered body. Therefore, it is necessary to perform the surface polishing process using the lapping machine at the last stage for manufacture of the heat radiator board. This causes complications in the manufacturing process of the heat radiator board, which is lead to an increase in the manufacturing cost. Further, organic binders are used to improve fluidity, mold ability, and shaping ability of material powders for use in formation of the sintered body. For this reason, it is necessary to perform a degreasing process. If the degreasing process is performed insufficiently below the required level, carbides easily adhere to the surface of the sintered body to fill its vacancies. This raises the difficulty in operating infiltration (or impregnation) of high thermal conductive materials.
Further, vacancies in the sintered body tend to contain non-infiltrated (or non-impregnated) portions of high thermal conductive materials, so that pinholes are easily formed on the surface of the sintered body. If a plating layer is formed on the surface of the sintered body having pinholes, plating blisters are easily formed. Therefore, it is difficult to produce a heat radiator board having a high quality realized by good plating. After infiltration (or impregnation) of high thermal conductive materials, a large amount of excess impregnated materials adheres to the surface of the sintered body. Therefore, it is necessary to perform the surface polishing process after the excess impregnated materials firmly adhering to the surface of the sintered body are removed by the polishing process and the like. This increases the number of finish processing steps in manufacture of the heat radiator board, which may lead to an increase in the manufacturing cost.